The present invention relates to discs with radially-extending blades and a method of producing such discs. Such discs are used in pumps, especially turbo-molecular pumps.
A turbo-molecular pump is composed of rotor and stator discs which are disposed alternately one behind the other and comprise rings of blades or vanes. The pump action is based on the fact that impulses in the pumping direction are transmitted to the molecules of the gas to be pumped by the blades of the rotor discs. The two important pump characteristics, namely pumping speed and pressure ratio, depend very much on the speed of rotation of the rotor discs. The pumping speed increases linearly with the speed of rotation and the pressure ratio does so exponentially. In order to achieve an optimum pump action therefore, the rotor must rotate at the highest possible speed. As a result, heavy demands are made on the blades of the rotor discs with regard to their geometry, mechanical strength and stability.
Blades for discs of turbo-molecular pumps and/or methods of producing such blades or discs are described, inter alia, in the following publications:
DE-PS 10 15 573 PA1 DE-PS 15 03 704 PA1 DE-PS 20 35 063 PA1 DE-PS 20 50 555 PA1 DE-AS 22 29 724 PA1 DE-AS 26 54 055 PA1 DE-AS 27 17 366 PA1 DE-OS 29 23 632 PA1 FR-PS 12 97 182 PA1 FR-PS 13 06 013
Becker, Nesseldreher: Neue Hochleistungsscheiben fur Turbo-Molekularpumpen-Vakuum-Technik 23 (1973), 1 pages 12-15.
The blades proposed in these publications have a rectangular, rhombus-like or triangular cross-section or profile.
A prerequisite for optimum pump characteristics is a minimum thickness of the blades. This is due to the fact that, because of the desired high speed of rotation, the centrifugal force which acts on the blades, the roots of the blades and on the inner diameter of the rotor discs must be restricted to a minimum for reasons of strength. Furthermore a limited blade thickness of rotor and stator discs is desired in order to make the suction space between the blades as large as possible which is one of the prerequisites for a high pumping speed.
A limited blade thickness however, has a disadvantageous effect on the stability of the blades. As a result of resonances, for example, which occur at certain speeds of rotation during the running up to speed or running down of the rotor, oscillations may occur which can lead to contact between the blades and the stator discs, which generally leads to destruction of the pump.
Deformation and oscillations of the blades also occur in the event of venting phenomena which produce heavy forces in the axial direction.